Tag Archives: steel tornado shelter

“If you are not underground you will not survive!”

As storm shelter designers we constantly hear the statement made that “You cannot survive an EF4 or EF5 tornado unless you are underground”. This information couldn’t be further from the truth. We now know there are codes in place to assist designers and engineers in designing above ground shelters that will withstand an EF5 tornado and provide near-absolute protection to those seeking refuge during the storm.

There have been several above ground shelters in the past few years which have performed quite well in tornados in Oklahoma. To my knowledge there has only been one fatality in an above ground shelter that was constructed per the FEMA P-320 EXCEPT it was found to have had a non-tested door assembly installed. (Reference Schultz Squared Architects’ earlier blog for this information.)

While there is nothing wrong with the idea of being underground in the event of the tornado, I would like to point out some of the potential, problems with underground shelters.

Bobber Effect: This one is pretty scary, and with the large amounts of rain and the ground becoming saturated last summer in Oklahoma there were numerous tornado shelters that actually floated out of the ground just like a fishing bobber in water!
Water infiltration: Whenever a portion of any building is put underground, there is always a potential for ground water to find its way into the building either through surface water or ground water. For obvious reasons the last thing that you need is a tornado shelter that has doubled as an underground swimming pool! Many of the residential underground shelters do not provide waterproofing and/or drainage system around the exterior (similar to a foundation drain around your basement that dumps into your sump) and over time, this could become an issue for you.
Exiting: If you are underground in a debris carrying tornado and it drops your house/building on the door, does anyone know to look for you as you may become trapped for some time before being “dug” out. If that’s the case let’s just hope the torrential rains do not follow the tornado or…see issue number 2.
Accessibility: In a residential shelter stairs are an acceptable method of accessing an underground tornado shelter, for the elderly or physically impaired person whether temporarily or permanently this may not be feasible. In a community shelter wheelchair, accessibility route must be provided along with (2) exits from an underground space, which adds significant cost to the design of the facility.
Ventilation: The building code requires a certain amount of ventilation be provided for both residential and community shelters. Some of that ventilation should be near the floor and some near the roof of the shelter. This is much more difficult to accomplish in a community underground shelter. Another issue regarding ventilation/infiltration are hazardous vapors, anything that is heavier than air will have a tendency to find its way into your underground shelter, i.e. solvents, paint, gasoline, diesel, & kerosene, all of which could be disastrous if it finds its way to a flame source either internal or external to the shelter.

If you decide an underground shelter is right for your building/house do your research and make sure the designer/supplier is reputable and knows how to prevent the issues I have listed above because the tornado is not the only danger that may surround the event. Please don’t misinterpret this blog and believe I am telling you to avoid a well-designed/installed underground shelter as it may be the best place for you to be in a high wind event. Just understand what you are getting yourself into.

Thanks for following, and remember. Ask questions, do your research, and make an informed decision. The lives of you or others may depend on it. Written by Shauna Schultz, AIA

Tornado Shelters-The “3 ½” Concrete Slab Issue

The 3 ½” concrete slab issue is one that I have attempted to distance myself from because I spoke against it in the initial ICC-500 code committee and got resoundingly shot down by the rest of the group. Please allow me to explain the issue as I originally saw it and now that I know more, how it stands now.

Section 309.1.2.1 of the 2008 edition of the ICC-500 states”

Slabs on grade shall be designed for applicable loads in accordance with Section 301. Where a slab on grade is being used to resist loads, the minimum thickness shall be 3 ½ inches (88.9 mm) and the minimum steel reinforcement for slabs on grade resisting forces on the storm shelter shall be 6×6-W1.4 by W1.4 or No. 4 bars, 18 inches on center in either direction.

I was opposed to this language for one reason and one reason only. I knew when people (i.e. designers, engineers, manufacturers) read this, they believe that a 3 ½” slab will work for any type of shelter as long as it has the reinforcing indicated without the need to have it engineered. Low and behold, we have small residential shelters made of steel, wood, and other composite materials that are light weight being bolted to existing concrete slabs without doing any design work to confirm that the 3 ½” slab will work as required by the first sentence of the paragraph noted above! I knew it would happen!! I knew it!

I know of four competent, practicing structural engineers that have looked at this very issue and not a one of them could engineer a 3 ½” slab to work with these light weight shelters that experience the full wind load; NOT ONE!!! Why? BECAUSE IT DOESN’T WORK!!! The wind forces attempt to overturn these light weight shelters but doesn’t have enough “lead in its ass” to resist it so it depends on the dead weight of the concrete slab to resist the force. Problem is, the 3 ½” concrete slab doesn’t have enough “lead in ITS ass” to resist the force either!

With that said, there are exception to every rule. If you have one of the heavier shelters, i.e. fully reinforced masonry, and/or concrete, there may be enough dead weight in the walls and roof of the shelter to resist the overturning force. There are stipulations to this in the ICC-500, and only engineering will determine this. Shelters that are located in basements and do not see the full wind load may also be an exception, but again, this needs to be verified!

The latest version of the ICC-500 which has yet to be released has reworded this paragraph to hopefully emphasize the need for engineering these slabs. The problem is going to be, will pre-fab shelter manufacturers pick up on this and comply? Do you think Home Depot which sells shelters on the internet is going to make sure that you have an engineered slab on which to install your newly purchased shelter? I hardly doubt it!

So when you see manufactured shelters that say they can be bolted to an existing slab without any type of engineered verification, STAY AWAY! It is a must that there is enough dead load to keep the shelter from overturning, where ever that dead load comes from. It could be footings/foundations, thick floor slab (like 2’ thick), heavy walls, and/or roof structure. But bolting to an existing slab that is 3 ½” thick, or 4” thick is not going to cut it! You probably did not purchase a tornado shelter, you purchased a bad carnival ride! And one that could kill or serious injure you and your family.

Be careful out there!

Post by Corey Schultz, AIA, LEED BD+C

Tornado Shelter Walls versus Kinetic Energy and Deformation

Growing up, my Dad had sitting on his office desk a little device called Newton’s Cradle. It had 5 suspended steel balls hung from a frame. You raised one of the end balls, let it go and when it hit the others, the one on the opposite end would fly up. This device represents Kinetic Energy. I didn’t understand it at the time but for a kid, it was fun to play with!

Now, using that same principal, think about one’s head leaning up against the exterior wall of a concrete or masonry tornado shelter. On the opposite side of the wall, a 15 pound 2×4 traveling at 100 mph strikes right where one’s head is against the wall. Get the picture?

Along those same lines, the ICC-500 states that a maximum of 3” of permanent deformation of an interior surface is allowed. 3”! So, you are sitting in one of these steel shelters where the only thing between you and the tornado debris is a sheet of steel that can deflect up to 3” and the shelter manufacturer has so conveniently put a built-in bench for you, on the exterior wall of the shelter with the interior surface of the shelter as you back rest. A backrest that can have a permanent deformation of 3”! How do you think one’s skull, spine, and/or shoulder blades are going to react to that potential 3” deformation?

The long and short of this is regardless of the shelter or the shelter material, during a tornadic event, shelter occupants should stay at least 3” away from the exterior walls of the shelter.